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    Plan

    Plan détaillé Texte intégral Introduction General characteristics of fish communities Biology and adaptive strategies Functional role of lagoon and estuarine ecosystems Conclusions Auteur

    The inland water fishes of Africa

    Ce livre est recensé par

    Précédent Suivant
    Table des matières

    Fish communities in estuaries and lagoons

    Jean-Jacques Albaret

    p. 417-440

    Résumé

    Estuaries and lagoons, along with mangroves and maritime marshes, occupy a considerable area along the coast of the African continent. Often subjected to strong marine influence, both in terms of their physical structure and their bio-ecological functioning, lagoon and estuarine environmental systems (abbreviated later as LEE) are nonetheless generally attached to the field of continental waters. In fact, the diversity of situations makes it difficult, if not impossible, to classify them as either marine or continental, and the term “interface environment” perfectly captures their nature and environmental functioning.

    Texte intégral Introduction General characteristics of fish communities Lagoon or estuarine environments: strong biodiversity or species-poor systems? Factors influencing or regulating diversity Size of the system Extent, number, diversity, and permanence of interfaces Diversity of available habitats Population potential (diversity of colonization sources) Biogeography Other factors that may influence species richness Classification of species in ecological categories Different types of estuarine communities Organization of communities Factors for community structuring Structuring role of environmental factors Importance of interspecific relationships Stability-instability Biology and adaptive strategies Reproduction and breeding strategies Space occupation strategies Food webs and resource use strategies Functional role of lagoon and estuarine ecosystems Exportation of organic matter and energy Nursery Conclusions Auteur

    Texte intégral

    Introduction

    1Dufour et al. (1994) point out that the plethora of definitions proposed to express the concept of “lagoon” indicates that there are no universally-accepted criteria for differentiating them from bays, estuaries, coastal marshes, and other parts of the coastal landscape (Mee, 1978). According to the outline proposed by Davies (1973), there is a continuum within the broad category of estuarine environments (figure 22.1). One end of the spectrum contains lagoons of marine origin (wave action), sheltered behind a sedimentary belt formed by relatively large-sized particles (sand). On the other end of the spectrum can be found deltas, resulting from the action of rivers rather than marine activity, and characterized by the small size of sedimentary particles (silt). Between these two, there are different types of environments that display a mix and a gradation of the two extremes of coastal environments. A large number of type classifications of lagoon and estuarine environmental systems have been proposed, each favouring a specific approach: geological, morphological, hydrodynamic, physico-chemical (water quality, particularly salinity levels), bio-ecological. A typology of LEEs can thus not claim to be either exhaustive or multipurpose. To give an example of a morphological and hydrodynamic approach, we can mention the classification proposed by Lankford (1977) which prioritises the dynamics of formation, with sub-classes taking partly into account a given system’s opening to the marine environment (figure 22.2), and that of Nichols and Allen (1981) which distinguishes two main dynamic factors that determine both the evolution and the function of lagoons (figure 22.3):

    • the effect of tidal currents and continental discharge;
    • the effect of waves and coastal current.

    FIGURE 22.1. Schematic representation of the continuum from lagoons to deltas (adapted from Davies (1973).

    Image

    2Based on the dominant influence of each of the two factors, lagoons can be classified into four principal types (Nichols & Allen, 1981) (figure 22.3).

    FIGURE 22.2. Different lagoon types based on formation process (from Lankford, 1977).

    Image

    FIGURE 22.3. Four lagoon types based on the importance of dynamic and energetic factors (from Nichols & Allen, 1981).

    Image

    3For various reasons that essentially have to do with their apparent complexity (if only for the mixture of fauna of different origins) but also the difficulty of defining and demarcating these fluctuating systems in space and time, the study of these systems as ecosystems in their own right is relatively recent, particularly in the intertropical area. Moreover, available information remains fragmented and often sparse. Nonetheless, in recent years LEEs have become the focus of growing scientific interest, tied in large part to the development of multiple human activities that are performed within or near them, such as fishing, aquaculture, tourism, urban and port planning. In addition, their generally small dimensions (compared with oceans and large fluvial basins), the intensity of physical and biological gradients, and the diversity and scope of hydro-climatic variations make them desirable sites for the application of ecological methods and theories.

    4Because of its halieutic and aquacultural applications, ichthyology has been one of the most frequently-tackled scientific fields.

    5Two large areas that are particularly rich in LEEs, West Africa and Southern Africa (sometimes referred to as sub-continents), have been the subject of extensive ichthyological investigations and will provide most of the information and bibliographical elements mentioned in this chapter (figures 22.4 and 22.5). It can be noted that in both cases, the scientific approach and reasoning were essentially the same. Initially research focused on the systematic inventory and taxonomy of species, which sometimes allowed a biogeographical approach. Studies on the biology and ecology of the major species developed at the same time. Subsequently, synecological research developed, as scientists attempted to describe and sometimes explain the ecological functioning of the communities of specific systems: the Ébrié lagoon, Casamance, Lake Togo, Sine Saloum, Fatala estuary for West Africa; St Lucia System, Lake Nhlange (Kosi System), Poelela Lagoon for Southern Africa. The acquisition of this body of knowledge, which will be referred to extensively in this chapter, now allows a twofold comparative approach – firstly geographic, with the comparative analysis of faunistic and functional characteristics of several systems, but also diachronic, making it possible to follow and understand the evolution of an estuary system subjected to considerable changes (particularly climate-related) to which it is sometimes possible to make a connection with changes in the composition and structure of communities. In this chapter, we will attempt to provide the elements that explain what an estuarine species is, what its origin may be, how ichthyological communities are formed and organized and to point out the essential factors (biotic or physical) that influence their structure. We will examine the vital strategies used by species to take the greatest advantage of these supposedly stressful and inhospitable environments. Some pressing current issues regarding LEEs will also be discussed, particularly the debate regarding their biodiversity (is it high or not?) and the vital question – as their survival sometimes depends on it – of the ecological and economic role of these systems. We will conclude by discussing the concepts of the fragility and sturdiness of these environments and the threats that face them.

    FIGURE 22.4. Location of some of the main LEEs studied in West Africa (based on Diouf 1996).

    Image

    FIGURE 22.5. Coastal lagoons of south-east Africa (based on Blaber, 1985).

    Image

    General characteristics of fish communities

    6By definition, LEEs are found at the interface of continental and marine areas. Like the physical system, the fauna and flora of these environments are influenced by the adjacent systems based on an equilibrium that varies according to situation. The question then arises whether the faunistic compositions that occupy these ill-defined environments present a certain degree of structure and meet some general organizational principles, or if on the contrary they are simple, more or less random aggregates of opportunistic fish originating from either the adjacent continental plateau or from continental tributaries.

    Lagoon or estuarine environments: strong biodiversity or species-poor systems?

    7Depending on the author, lagoon and estuarine environments are presented as species-poor environments (Remane & Schlieper, 1971; Kiener, 1978; Day & Yañez-Arancibia, 1985) or as ecosystems that are rich in species (Albaret, 1994; Albaret & Diouf, 1994; Baran, 1995). The main cause of this divergence is the absence of a consensus on the definition of estuarine ichthyofauna during species inventories (what is an estuarine species; what are the limits of the estuary; what should be counted?). In the first case, generally the only species taken into account are those that complete their entire life cycle in estuaries and lagoons, species that are exclusively confined to brackish environments. Species of marine or continental origin that are found temporarily in LEEs, or those that can complete their life cycles in or out of the estuary environment are excluded from inventories. Yet for some of these species, lagoons and estuaries serve an extremely important function (“nursery” role) that is sometimes indispensable for the completion of the life cycle (Wootton, 1992; Beckley, 1984; Ross & Epperly, 1985; Day et al., 1989). Moreover, the interactions (predation, competition) of these migratory species with resident ichthyofauna are an important contributor to the structure of fish populations (Monteiro et al., 1990; Blaber, 1985). If we are to study how these systems function, it thus seems necessary – going beyond inventory and classification – to include migratory and amphibiotic species in estuarine ichthyofauna and discuss the notion of species richness in LEEs while taking them as open systems.

    8An examination of the structure of fish communities in estuaries and lagoons around the world shows that, in general, only a few species contribute to most of the biomass of these otherwise species-rich systems (table 22.I and 22.II) (Kennish, 1990; Yañez-Arancibia et al., 1985; Chao et al., 1985; Horn & Allen, 1985; Whitfield, 1994a; Kromer et al., 1994; Albaret, 1987; Baran, 1995; Diouf, 1996).

    TABLE 22.I. Species richness of ichthyofauna in different countries of West Africa (adapted from Baran, 1995).

    Image

    TABLE 22.II. Species richness of ichthyofaunal of some LEEs around the world (Diouf, 1996).

    Environment type

    Country

    Specific richness

    Source

    Africa

    Senegal

    estuary

    Senegal

    111

    Reizer, 1988; Diouf et al., 1991; Kébé et al., 1992

    Sine Saloum

    inverse estuary

    Senegal

    114

    Séret, 1983

    Gambia

    estuary

    Gambia

    89

    Daget, 1960; Dorr et al., 1985

    Casamance

    inverse estuary

    Senegal

    86

    Pandaré & Capdeville, 1986;
    Pandaré, 1987; Albaret, 1987;
    Pandaré & Niang, 1989; Badji, 1990

    Rio Buba

    ria

    Guinea-Bissau

    92

    Kromer et al., 1994

    Fatala

    estuary

    Guinea

    102

    Baran, 1995

    Ébrié lagoon

    lagoon

    Côte d’Ivoire

    153

    Albaret, 1994

    Aby lagoon

    lagoon

    Côte d’Ivoire

    82

    Charles-Dominique, 1994

    Lagos lagoon

    lagoon

    Nigeria

    79

    Fagade & Olanyan, 1974

    Niger

    delta

    Nigeria

    52

    Boeseman, 1963

    Ogowe

    estuary

    Gabon

    66

    Loubens, 1966

    Santa Lucia

    lagoon

    South Africa

    108

    Blaber, 1988

    Poelela

    lagoon

    South Africa

    12

    Blaber, 1988

    Nhlange (Kosi)

    lagoon

    South Africa

    37

    Blaber, 1988

    Sibaya

    lagoon

    South Africa

    18

    Blaber, 1988

    Swarvlei

    lagoon

    South Africa

    25

    Blaber, 1988

    Richard’s Bay

    lagoon

    South Africa

    74

    Blaber, 1988

    Morrumbene

    estuary

    Mozambique

    114

    Day, 1974

    Pangalanes

    8 small lagoons

    Madagascar

    10 to 43

    Lasserre, 1979

    America

    Teacapan-Agua Brava

    lagoon

    Mexico

    75

    Flores-Verdugo et al., 1960

    Terminos Lagoon

    lagoon

    Mexico

    122

    Yañez-Arancibia et al., 1980

    Santa Lucia

    estuary

    Mexico

    83

    Chavez, 1979

    Nichupte

    lagoon

    Mexico

    37

    Chavez, 1979

    Tuxpan

    lagoon

    Mexico

    126

    Chavez, 1979

    Alvarado

    lagoon

    Mexico

    71

    Chavez, 1979

    Laguna Madre

    lagoon

    Mexico

    111

    Chavez, 1979

    Tamiahua

    lagoon

    Mexico

    49

    Chavez, 1979

    Mandinga

    lagoon

    Mexico

    24

    Chavez, 1979

    La Mancha

    lagoon

    Mexico

    42

    Chavez, 1979

    Tabasco

    lagoonal system

    Mexico

    62

    Resendez-Medina, 1979

    Tamiahua

    lagoon

    Mexico

    56

    Resendez-Medina, 1979

    Zontecopopan

    lagoon

    Mexico

    50

    Resendez-Medina, 1979

    Nicoya Gulf

    coastal mangrove

    Costa Rica

    61

    Philips, 1983

    Punta del Este

    coastal mangrove

    Cuba

    55

    Valdes-Muñoz, 1983

    Laguna Joyuda

    lagoon

    Puerto Rico

    41

    Stoner

    Belle-Plaine

    lagoon

    Guadeloupe

    25

    Louis et al., 1985

    Mancha à Eau

    lagoon

    Guadeloupe

    24

    Louis et al., 1985

    Fort-de-France bay

    coastal mangrove

    Martinique

    87

    Louis et al., 1985

    Cayenne River

    estuary

    French Guiana

    59

    Tito de Morais & Tito de Morais, 1994

    Barrier reef complex of Belize

    reef

    Belize

    87

    Sedbery & Carter, 1993

    Jaguaribe

    estuary

    Brazil

    86

    Oliveira, 1976

    Huizache-Caimanero

    lagoon

    Mexico

    44

    Warbuton, 1978

    Tijuana estuary

    estuary

    USA (South California)

    21

    Nordby & Zedler, 1991

    Los Penasquitos Lagoon

    lagoon

    USA (South California)

    13

    Nordby & Zedler, 1991

    Wells estuary

    estuary

    USA (Maine)

    24

    Ayvazian et al., 1992

    Waquoit Bay

    Bay

    USA (Massachusetts)

    46

    Ayvazian et al., 1992

    Asia

    Pagbilao

    coastal mangrove

    Philippines

    128

    Pinto, 1988

    Klang-Langat

    delta

    Malaysia

    119

    Chong et al., 1990

    Oceania

    Labu Estuary

    estuary

    Papua New Guinea

    38

    Quinn & Kojis, 1986

    Moreton Bay

    coastal mangrove

    North Australia

    112

    Weng, 1988

    Trinity inlet system

    estuary

    North Australia

    91

    Blaber, 1980

    Dampier Bay

    coastal mangrove

    North Australia

    113

    Blaber, 1980

    Leanyer Swamp

    coastal mangrove

    North Australia

    38

    Davis, 1988

    Alligaror Creek

    estuary

    North Australia

    128

    Robertson & Duke, 1990

    Embley estuary

    estuary

    North Australia

    197

    Blaber et al., 1989

    Botany Bay

    coastal mangrove

    North Australia

    46

    Bell et al., 1984

    Swan River

    estuary

    West Australia

    71

    Loneragan et al., 1989

    Deama

    coastal mangrove

    New Caledonia

    75

    Thollot, 1989

    Europe

    Ria Aveiro

    estuary

    Portugal

    55

    Rebelo, 1992

    Ria Formosa

    estuary

    Portugal

    67

    Monteiro, 1989

    Étang de Thau

    lagoon

    France

    53

    Bach, 1985

    Mauguio

    lagoon

    France

    71

    Bouchereau, 1994

    9The diversity in the definition of limits both upstream and downstream of LEEs (Caspers, 1954; Rodriguez, 1975; Fairbidge, 1980; Day, 1981; Baran, 1995), is also a source of controversy regarding the species richness of these systems.

    10In a given biogeographic province, the species richness of LEEs in West Africa is comparable to that of rivers and never less than a third of the number of species found at sea on the neighbouring continental plateau (Baran, 1995) (table 22.I). At the global level, we can note that the richness of Africa’s estuaries and lagoons is greater overall compared with other LEEs (table 22.II).

    Factors influencing or regulating diversity

    11The LEEs of West Africa thus play temporary or permanent host to a large number of species. This richness can be traced to certain characteristics of these ecosystems.

    Size of the system

    12In the ecology of aquatic systems, it is common for scientists to describe the species richness of an ecosystem according to its extent (see chapter Species richness of fish communities). The main condition for the application of this model stipulates that ecosystems should form isolated ensembles such that on their borders, their fauna does not undergo any pressure from neighbouring fauna. Yet one of the fundamental characteristics of LEEs is, indeed, to allow the integration of elements from adjacent systems into the animal communities that they host. Another essential characteristic of these ecosystems is that they rely primarily on exchanges (of matter, energy, and biomass) with adjacent systems. While their dimensions have an influence on their species richness, the area/species richness model does not apply to LEEs.

    Extent, number, diversity, and permanence of interfaces

    13The reasons for the extent of species richness in a lagoon or estuary must be treated as having more to do with the extent, diversity, and permanence of its interfaces with adjacent systems (Albaret, 1994). The moment and duration of their connection with the ocean also play an essential role on the diversity and abundance of fish in a given lagoon or estuarine system. LEEs that have a permanent connection to the ocean show greater species richness compared with systems that have temporary connections (Whitfield & Kok, 1992).

    Diversity of available habitats

    14The morpho-edaphic (shape of water body, depth, nature of bottom) and bankside vegetal variety, heightened by the diversity of hydrological and hydroclimatic situations, leads to a great diversity of habitat types available to fish communities in the LEEs. (Blaber, 1974; Branch & Grindley, 1979; Marais 1982; Whitfield, 1983; Cyrus & Blaber, 1987a, b, c; Albaret 1987; Diouf 1996.).

    Population potential (diversity of colonization sources)

    15This high “hosting capacity” also corresponds to very high colonization potential from coastal marine, estuarine, and continental species.

    16A large proportion of the fish species making up the neritic communities of the African coasts is liable to penetrate into neighbouring estuaries and lagoons for varying periods and at different life stages.

    17Estuarine species, which are euryhaline and have generally low endemicity, can (little by little, and at another time scale) also colonize new LEEs via the sea.

    18Continental species, generally less numerous than the others, come from rivers that are tributaries of the LEEs.

    19The species of the first two groups have a vast biogeographic distribution whereas the majority of continental species have a more restrained area of distribution. This contributes to the regional, even local (endemic species) specificity of estuarine communities.

    Biogeography

    20Biogeography also plays an important role in determining the “availability” of species for recruitement in a given estuary (Blaber, 1981).

    21Whitfield (1994a, b) notes that in general, tropical and sub-tropical estuaries have more species richness than temperate systems owing mainly to the greater richness of fauna associated with habitats that are close to the equator, be they marine or riverine. In South Africa, Wallace and van der Elst (1975) have shown that there is a decrease in ichthyological diversity when one moves from the sub-tropical estuaries of the Natal (Indian ocean) to the “temperate-warm” ones of the Cape’s coast then the “temperate-cold” ones of the Atlantic shoreline.

    Other factors that may influence species richness

    22Other factors, often interdependent, can be mentioned: the strength and duration of floods, (Marais, 1982), physico-chemical characteristics of water and their spatio-temporal variations (Blaber, 1974; Whitfield et al., 1981; Cyrus & Blaber, 1987a, b, c; Albaret, 1987; Diouf, 1996), trophic richness and resource availability (Marais, 1984; Whitfield, 1988); presence, abundance, and health state of mangroves (Diouf, 1996). The composition of LEE communities is also modified, sometimes considerably, by human interventions such as the construction of engineering works (ports, dikes, dams, etc.), creation of connections to the ocean, halieutic and aquacultural activities, pollution from urbanisation and industrialization as well as agricultural activities (Blaber et al., 1984; Plumstead, 1990; Albaret & Diouf, 1994).

    23Finally, we should note the essential role of historical factors (at various time scales) in the creation of fish communities as we know them today. How, indeed, would it be possible to understand the current organization of the highly saline Casamance and Sine Saloum estuaries (Senegal) if we were not aware of past climate evolution and the recurrent rainfall deficits typical of this Sahelian region?

    24On another scale, it can be useful to remember that the existence of LEEs, tied to major climatic changes and ocean level fluctuations, have existed for a relatively short time compared with lakes, rivers, seas, and oceans. To a certain extent we can thus think of them as being ephemeral (on a case-to-case basis, as there will always – more or less – be lagoons and estuaries regardless of sea level) on a geological (and evolutionary process) time scale. This is probably one of the reasons for the low level of speciation in LEEs (as indicated by the low number of species per family) compared with other aquatic systems and, consequently, the relatively low number of true, uniquely estuarine species.

    25As an example, the Ébrié lagoon, with more than 150 species, is one of the LEEs with the greatest species richness, and combines many factors that encourage population diversity (Albaret, 1994). Located near the equator (5 ° North), it is the largest lagoon system in West Africa (566 km2), possessing interfaces with a wide range of aquatic ecosystems: forest rivers, Sudanese-Sahelian river, other lagoons, mangrove, marshes, a continental shelf with over 300 fish species. The connection with the ocean occurs mainly through the Vridy canal, an artificial permanent opening (width: around 300 m, average depth: 15 m) allowing the transit of considerable water masses (annual entry 38.109 m3, annual discharge 50.109 m3) (Varlet, 1978). The extreme variety of landscapes and the diversity of hydro-climatic situations round out the list of factors encouraging the temporary or permanent hosting of many species from different origins, and showing a wide range of ecophysiological adaptations and biological cycles.

    Classification of species in ecological categories

    26Undoubtedly more than for any other type of aquatic system, specialists have run into the problem of defining and classifying estuarine and lagoon communities, an essential step for describing and comparing communities, then for investigating certain aspects of their functioning. Typologies based in the main on the osmoregulatory capacities of species have been proposed (Kiener, 1978) but these have turned out to be largely unsatisfactory when put in practice. A classification that takes not only the degree of euryhalinity into account, but also the characteristics of each species’ bio-ecological cycle (distribution and regularity, breeding sites and conditions, place in the trophic network, abundance and location of ecophases), makes it possible to gain a better grasp of the origin, nature, and certain functional aspects of ichthyological communities. Such classifications, undergirded by the concept of estuarine dependence and adapted to the study of vast regions, have been proposed for the subtropical and temperate estuaries of southern Africa (Whitfield, 1994a, b) and the LEEs of West Africa (Albaret, 1994; Albaret & Diouf, 1994). The Whitfield classification has five main categories, three of which are subdivided, for a total of nine categories or sub-categories. The one proposed by Albaret (figure 22.6) includes eight bio-ecological categories, which from a central point of strictly estuarine species, splits unevenly on two gradients of continental and marine affinities. Four of these groups (strictly estuarine, estuarine of marine origin, estuarine of continental origin, marine-estuarine species), composed of largely euryhaline and eurybiotic species, make up the basic elements of the ichthyocenosis of the LEEs of West Africa. About thirty of these species constitute the core of communities owing to their ubiquity, their abundance (or at least their regularly), and the essential role they play in the halieutic economy and ecology of these systems (table 22.III). The other groups comprise species that are variously referred to in literature as “complementary”, “accessory”, “rare”, or “occasional” species. They are found on the extreme ends of the marine and continental axes, their number and diversity varying according to type, size, and hydrological and hydroclimatic characteristics of the system in question.

    TABLE 22.III. List of species common to West African LEEs (Diouf, 1976).

    Arius latiscutatus

    Gerres nigri

    Citarichthys stampflii

    Porogobius schlegelii

    Caranx senegallus

    Pomadasys jubelini

    Chloroscombrus chrysurus

    Liza dumerili

    Trachinotus teraia

    Liza falcipinnis

    Hemichromis fasciatus

    Liza grandisquamis

    Sarotherodon melanotheron

    Mugil curema

    Tilapia guineensis

    Galeoides decadactylus

    Ethmalosa fimbriata

    Ilisha africana

    Sardinella maderensis

    Pseudotolithus elongatus

    Cynoglossus senegalensis

    Pseudotolithus senegalensis

    Dasyatis margarita

    Scomberomorus tritor

    Elops lacerta

    Sphyraena afra

    Eucinostomus melanopterus

    Monodactylus sebae

    Different types of estuarine communities

    27In the diverse and varied LEEs of West Africa, the different ecological categories described are more or less well-represented (figure 22.6) and we can schematically describe the theoretical evolution of fish populations based on hydroclimatic characteristics (Diouf, 1996) (figure 22.7).

    FIGURE 22.6. Main ecological categories in fish communities of West African LEEs.

    Image

    LPA: lagoonal populations abundance (# rare, ## abundant, ### very abundant);
    LR: lagoonal reproduction;
    LD: lagoonal distribution;
    W: wide;
    L: limited;
    S: seasonal;
    C: continuous;
    E: euryhalinity (# low, ## high, ### quasi-total)

    28All the ecological categories are represented in certain LEEs, largely open to both marine and continental domains (type E). This is the case of the Ébrié lagoon, of the Senegal River estuary before the commissioning of the Diama dam, and of the estuaries of the Gambia and the Niger Rivers. Type M is characterized by the reduction of continental influence and the predominance of oceanic influence. This situation is seen in the Rio Buba in Guinea-Bissau, for example. In the inverse estuaries (type I) of Casamance and Sine Saloum (Senegal), the “centre of gravity” of the population shifted towards the marine side with a near-disappearance of the continental component (one or two species of group E. c. can remain). In the case of certain lagoons or certain lagoonal estuaries, the shift occurred in the other direction and the categories of the marine affinities axis are absent (M. o., M. a., M. E.) or hardly represented (E. m). Lake Togo during its closed-off period (Laë, 1992a) and the Potou and Aghien lagoons in Côte d’Ivoire (Albaret, 1994) are good examples of this type of situation (type C).

    FIGURE 22.7. Theoretical evolution of LEEs in Western Africa on the basis of their hydroclimatic characteristics (after Diouf, 1996); RC: resistance community.

    Image

    Organization of communities

    29The recruitment of fish in LEEs has three distinct sources: marine species and continental species that penetrate the estuarine area at a more or less advanced stage of their development, and juvenile forms of species that reproduce there obligatorily, regularly, or occasionally. From these different population sources, fish communities organize themselves according to the bio-ecological cycles of species (first of all in terms of their eco-physiological aptitudes), the biotic capacity of the system (in particular trophic availabilities), and interspecific relationships (spatial competition, trophic competition, predation, etc.).

    30The great diversity and extreme variability of LEEs corresponds to very uneven degrees of organization and stability. Some communities are characterized by high species richness but highly unequal abundance distributions. These types of communities are characterized by the marked dominance of a very small number of species (or even a single species, generally a pelagic Clupeiform one, Ethmalosa fimbriata or Sardinella maderensis in West Africa) and correspond to juvenile communities undergoing constant renewal and with high biological activity (production, life cycles, energy transfer). Conversely, other communities with often lower species richness but a more even distribution of abundance reflect greater environmental stability and a higher degree of maturity and structure of the communities in place.

    Factors for community structuring

    31The species composition and structure of fish populations in a given location is a result of the action of a series of ecological filters (Wootton, 1992). The most obvious and striking characteristic of LEEs is without a doubt the high spatio-temporal variability of the aquatic environment. Therefore among these filters, environmental factors are generally considered to play a crucial role in the organization of fish communities in these systems that are considered highly “physically controlled”. When interspecific relationships have been studied, this was most often owing to their intrinsic interesting nature and not for the purpose of understanding the structure of communities. Hence, in some cases, an overestimation of the role of physico-chemical parameters (particularly salinity) and an erroneous or incomplete identification of factors that structure the ichthyocenosis have been achieved. It is thus important, in the quest for explanatory hypotheses accounting for the structure of fish communities in LEEs, to take into account the roles of the hydrobioclimate and of inter-species relationships simultaneously.

    Structuring role of environmental factors

    32Salinity, temperature, turbidity, and dissolved oxygen concentration have a strong influence on the composition, distribution, and abundance of communities in estuarine waters (Kennish, 1990). Diouf (1996) points out that for some fifty studies carried out in estuarine waters, turbidity (in 75% of cases) is one of the most frequently mentioned structuring factors for populations, followed by salinity (58%) and temperature (40%). Other factors are trophic richness (30%), nature of sediments (24%), depth (12%), current (11%) and finally oxygen (10%). However, while a wide variety of structuring factors are generally taken into account, a closer examination of their correlations show that the main ones are turbudity, salinity, and temperature; other factors are often no more than co-variables (McLusky, 1993).

    Importance of interspecific relationships

    33Sedentary and migrant (adult and juvenile) fish species living in LEEs may reach very high densities, which often results in strong intraspecific competition (Wootton, 1992). In extreme cases, the youngest stages are cannibalized.

    34As juveniles of many large species have the same dietary requirements as adults of small species, LEEs are the venue for strong interspecific competition. However, the spatial heterogeneity and temporal variability of the environment may limit such competition as a result of habitat segregation (Wootton, 1992; Blaber & Whitfield, 1977).

    35The intensity of predation is tied in large part to turbidity (Cyrus & Blaber, 1987b; Deegan & Thompson, 1985) since prey is hunted by sight for many species (Nikolsly, 1963; Hyatt, 1979; Wootton, 1992). As turbidity is often high in estuaries, predation there generally tends to be low. In some conditions, clear and deep waters, predation may increase (Blaber et al., 1985).

    36In estuaries and lagoons, it has been shown that predators rarely specialize in a single species (dietary opportunism). As soon as prey populations decline, predators select a closely-related species (Sykes & Manooch, 1978).

    Stability-instability

    37Fish communities in LEEs are often described as heterogeneous and unstable. This presumed instability (which sometimes earns them a reputation for fragility) must be revised after the findings of studies in West and Southern Africa. Granted, the level of structural organization is never very high in LEEs (primarily in zones that are close to connections with the ocean) and seasonal variations are very marked, in terms of both composition and structure of communities). Yet, in the Ébrié lagoon, Albaret & Écoutin (1990) have shown that at the scale of the annual cycle as well as at an interval of nearly twenty years, a group of species, mainly estuarine (E.c., E.s., E.m.) but also marine-estuarine (M. E.), is constantly present, which thanks to their ecophysiological performance constitute a remarkably stable base community in an environment that displays extreme variability at different levels. Moreover, it is interesting to note that these species are essentially the same as the ones forming the base community for all the LEEs of West Africa mentioned earlier. Beyond the stability of the base community, we can also note an evolution described as the “marinization” of the population which is seen as a consequence of the general rainfall deficit in the region during the period under study (Albaret & Écoutin, 1990). This heightened importance of the marine component within the populations can be felt more in the structure of the community (rank and frequency of species with an oceanic affinity in abundance distributions) than in their species composition which, aside from the inclusion of a few “accidental” marine species, fundamentally remains the same. The highly-marked seasonal variations of the community are induced by the changes in the environment, with the abundance and distribution of fresh water inputs at the fore. The cycle appears to be stable from one year to another, although modulated by the modalities of these inputs: degree and precocity of the rainy season locally, and of the flood of the Comoé river determined by rains in the Sudanian region.

    38Whitfield (1994a, b) points out that despite the instability or rather, the strong physical variability of the Southern African estuaries at various temporal and spatial scales, the species composition of ichthyologic communities of these environments is relatively stable, and fishes there have more or less predictable distribution and abundance patterns (Day et al., 1981). Moyle & Cech (1982) attribute this stability to four main conditions that apply to all Southern African estuaries (Whitfield, 1994a, b) and to West African LEEs:

    • regular distribution of populations throughout the gradients of the environment;
    • seasonal migrations of fishes in and outside the estuary;
    • dominance of only a few species within communities;
    • robust nature of food webs inside estuaries.

    39A fifth condition that seems essential can be added: the great physiological and ecological tolerance of species “using” estuaries (Whitfield, 1994b).

    40He also underscored the fact that LEEs differ from some other continental systems in that they have a low tendency to shift towards more mature animal and vegetal communities. In Lake Malawi, for example, the fish community is characterized by a growing proportion of specialized forms with progressively stronger interdependent relationships between species (Bruton, 1989). Meanwhile, the estuaries of South Africa, and African LEEs in general, are characterized by wide variations of abiotic conditions, the constant reorganization of biological communities and homeostatic mechanisms, and the frequent resetting of ecological successions to an earlier stage (Whitfield & Bruton, 1989).

    41Owing to the structural and functional complexity of LEEs, the stability and organization of fish communities, in the sense of the permanence of their species composition and structure, is subjected to two processes (Baran, 1995):

    • mobility of species that affects the degree of organization of biocoenoses,
    • ecophysiological plasticity of sedentary species that allow them to tolerate environmental variations and which encourage stability.

    42In the short term, that is at a seasonal level, the outcome of these two processes with antagonistic effects tends to encourage instability.

    43But, from year to year, the cyclical nature of climatological, hydrological, and hydrochemical factors that directly or indirectly determine the organization of communities through inter-species competition (Guiral, 1992) encourages a certain stability.

    44At the scale of one or two decades, this stability is also seen (Livingston, 1976; Whitfield, 1994a; Albaret & Écoutin, 1990) except in cases of rapid anthropogenic and/or natural changes in the environment (Albaret, 1987; Albaret & Écoutin, 1989; Albaret & Diouf, 1994).

    Biology and adaptive strategies

    45In fishes as in other organisms living in LEEs, physiological (Vernberg & Vernberg, 1976) and behavioural (Smith et al., 1966; McDowall, 1976; Day & Yañez-Arancibia, 1985; Day et al., 1989) adaptations have been developed to deal with the stress generated by the degree and speed of fluctuations in physico-chemical factors (McHugh, 1985; Deegan & Thompson, 1985; Day & Yañez-Arancibia, 1985; Whitefield, 1994a).

    46The strategies in play can be outlined in three broad categories: breeding strategies, space occupation strategies, and resource use strategies (Lévêque, 1994).

    Reproduction and breeding strategies

    47LEEs are often viewed solely as transit areas, “nurseries” for juvenile stages of fish whose adult life and reproduction occur in adjacent ecosystems, notably the marine environment. Only a few sedentary species, of small size and little ecological or economic significance, were believed to be capable of completing their full biological cycle in these systems whose strong variability and unpredictability are sometimes considered physiological obstacles to the reproductive process. In fact, studies conducted in West Africa (Albaret, 1987; Albaret, 1994; Diouf, 1996; Baran, 1995) show that lagoons and estuaries are the site of intense reproductive activity for a very wide range of species (table 22.IV). Reproductive activity is generally very widely spread out over time and space with, however, peaks of activity around the dry or rainy seasons depending on the marine or continental affinity of species. Environmental requirements, particularly for salinity, are generally low. The euryhalinity of most estuarine species at the time of reproduction is remarkable (Albaret & Diouf, 1994).

    TABLE 22.IV. List of species engaging in sexual activity in the LEEs of West Africa (Albaret & Diouf, 1994).

    Reproduction in lagoon

    Advanced maturation in lagoon

    Early stage of maturation in lagoon

    Brycinus longipinnis
    Aplocheilichthys spilauchen
    Chrysichthys auratus
    Chrysichthys maurus
    Dasyatis margarita
    Citarichthys stampflii
    Arius parkii
    Arius latiscutatus
    Cynoglossus senegalensis
    Eleotris daganensis
    Eleotris vittata
    Ethmalosa fimbriata
    Gerres nigri
    Gobionellus occidentalis
    Hemichromis fasciatus
    Liza falcipinnis
    Liza grandisquamis
    Monodactylus sebae
    Parailia pellucida
    Pellonula leonensis
    Pseudotolithus elongatus
    Pomadasys jubelini
    Porogobius schlegelii
    Sarotherodon melanotheron
    Schilbe mandibularis
    Strongylura senegalensis
    Tilapia guineensis
    Tilapia mariae
    Trachinotus teraia
    Trichiurus lepturus
    Tylochromis jentinki
    Yongeichthys thomasi

    Antennarius occidentalis
    Apogon imberbis
    Chromidotilapia guntheri
    Chrysichthys nigrodigitatus
    Eleotris senegalensis
    Fodiator acutus
    Hemiramphus balao
    Heterobranchus isopterus
    Hyporamphus picarti
    Liza dumerili
    Mugil curema
    Petrocephalus bovei
    Pseudotolithus senegalensis
    Schilbe mystus

    Brachydeuterus auritus
    Drepane africana
    Eucinostomus melanopterus
    Galeoïdes decadactylus
    Marcusenius bruyerei
    Plectorhynchus macrolepis
    Pomadasys incisus
    Sardinella maderensis
    Scomberomorus tritor
    Sphyraena afra

    48In the LEEs, reproductive strategies are aimed on one hand at minimizing the negative effects of environmental instability and on the other hand on combatting the dispersion of eggs, larvae, and embryos outside the estuary. Oviparous species have developed two “tactical choices” (Wootton, 1989). The first is based on maximizing the number of eggs released, and increases the probability that at least a fraction of the population will be transported to a favourable site in order to ensure recruitment (Liza spp., Ethmalosa fimbriata). The second tactic attempts to maximize the survival rate of eggs, larvae, and fry by limiting the number of eggs released and engaging in sometimes intensive parental care and/or seeking zones that are ecologically favourable to development. This, for instance, is the case of Arius spp, Sarotherodon melanotheron, and Tylochromis jentinki which practice mouth brooding, Chrysichthys spp whose clutches are attached and hidden in crevices or hollow pieces of wood, or Tilapia guineensis which build nests and guard the clutch and the fry (McKaye, 1989; Albaret & Diouf, 1994).

    49Viviparous (Clinus superciliosus in Southern Africa and Dasyatis margarita and D. margaritella in West Africa, for example) and ovoviviparous species (such as Syngnathus acus in Southern Africa and Enneacampus kaupi in West Africa) also have low fecundity. Embryos often reach a large size before hatching or release. Because of their large size, juveniles are less susceptible to predation or being carried away by currents (Whitfield, 1994a).

    50To reduce the risk of egg dispersal, groups with high estuarine affinity such as Atherinidae, Hemiramphidae and Gobiidae produce adhesive benthic eggs (Breder & Rosen 1966; Neira et al., 1988; Potter et al., 1990; Whitfield, 1990). Some species (mainly viviparous and ovoviviparous), sharply decrease or eliminate the initial larval stages and give birth to larvae that can swim well. It thus appears that the “precocial” reproductive strategy of some estuarine species can be traced to the fact that these fishes are attempting to limit the mortality risks associated with the uncontrolled dispersion of eggs and early larval stages (Whitfield, 1990). Other species such as Amniataba caudavittata, Nematalosa vlaminghi and Gilchristella aestuaeria (species from southwestern Australia and Southern Africa), spawn far from the river mouth, very high upstream in the estuary, thus reducing the risk that eggs are transported out to sea (Talbot, 1982; Chubb & Potter, 1984; Potter et al., 1990).

    51In most of the species reproducing in the estuary, the period of sexual activity is long if not continuous. This spread of the reproductive season can be interpreted as the guarantee of sufficient recruitment in all eventualities (Wallace, 1975; Baran, 1995).

    52The “eco-reproductive” plasticity of most strictly estuarine species (E. s.) and estuarine species of marine or continental origin (E. m. and E. c.) is the reason for the high variability in reproductive modalities and energy allocation for reproductive activity depending on changes in the environment (Pauly, 1975) and biological interactions (Barbault, 1981; Legendre, 1992). This is the case in particular of Ethmalosa fimbriata and Sarotherodon melanotheron which display reduced size at first maturity when the environmental conditions become extreme: pollution (Albaret & Charles-Dominique, 1982), hypersalinity (Albaret, 1987); life in a confined system (Legendre & Écoutin, 1991).

    Space occupation strategies

    53Heterogeneity, which increases the diversity of ecological niches, plays a fundamental role in the organization of communities (Barbault, 1992). It allows the maintenance of predator-prey systems by increasing the probability of survival of a fraction of the prey population which can colonize habitats that are not accessible to predators (Barbault, 1990).

    54Thus, in LEEs, many shoals, silt and sand bars, mangrove channels, as well as the root network of mangroves (stilt roots of Rhizophora and respiratory roots of Avicennias) are shelters that are accessible only to juveniles.

    55Moreover, the spatial segregation of species, related to heterogeneity, helps decrease the phenomena of inter-species competition (Blaber & Whitfield, 1977; Gorman, 1987; Wootton, 1992).

    56Species that use LEEs during their biological cycle demonstrate two occupation strategies to exploit to their advantage these nutrient-rich environments where predation is often low: migration and eco-physiological plasticity.

    57Migration is the solution adopted by a very large number of opportunistic species of marine origin and (to a lesser degree) species with a continental affinity that temporarily exploit the trophic richness of LEEs when the physicochemical (Guiral, 1992) and biological (Sykes & Manooch, 1978; Day & Yañez-Arancibia, 1982) conditions are favourable. The space occupation strategies of these migratory species aim in many cases to reduce the distances between marine breeding areas and estuarine and lagoonal nursery zones (Whitfield, 1990).

    58The second strategy is adopted by sedentary species (at the scale of one lagoon or one estuary) that adapt to the variability of the system owing to their strong adaptability and great eco-physiological tolerance. In the case of very significant environmental stress, representatives of this category are capable of surviving, thriving, and in some cases proliferating. The extreme abundance of Sarotherodon melanotheron, the only species present in the hyperhaline upstream section of the Casamance during periods of rainfall deficit is one of the most spectacular examples of this ability to adapt (Albaret, 1987).

    Food webs and resource use strategies

    59Fish diets and food webs have been the target of numerous investigations in both West Africa (Longhurst, 1957; Fagade & Olanyian, 1973; Pauly, 1975; Albaret 1994; Baran 1995; Diouf 1996.) and Southern Africa (Whitfield & Blaber, 1978a, b; Whitfield, 1980a, b).

    60LEEs are very productive ecosystems (Correll, 1978; Horn & Allen, 1985; Day & Yañez-Arancibia, 1982; Day et al., 1989). Among aquatic systems, only coral reefs are generally seen as being more productive (Whittaker & Likens, 1973; Whitfield, 1993). This productivity, which is reflected in the great availability of nutrients for fishes, stems from the number and diversity of nutrient sources (Day & Yañez-Arancibia, 1982): runoff (Day & Yañez-Arancibia, 1985), fluvial (Yañez-Arancibia et al., 1985) and marine (Cecchi, 1992) waters, remineralization of organic matter from river and submerged vegetation (William & Eric, 1972; Odum & Head, 1975; Twilley et al., 1986; Day et al., 1989; Alongi, 1988). In addition to all these advantages encountered by young fishes in estuarine systems, tropical zones are also characterized by the high production of mangrove formations (Odum & Head, 1975; Dor & Levy, 1984; Rodelli et al., 1984). While tree production is not directly consumed by fish, it allows the creation of a rich and complex microbial network (Guiral, 1994). This bacterial and fungal biomass, which mineralizes litter, as well as the algal, phytobenthic and epiphytic production occurring on mangrove roots (Rodriguez & Stoner, 1990) that stem partially from that biomass, make up particularly rich and varied nutrient resources owing to their zooplanktonic, micro-, meio-, and macro-benthic communities (Alongi, 1989). Trophic chains in LEEs are thus characterized by the diversity of primary producers (Day & Yañez-Arancibia, 1985; Flores-Verdugo et al., 1990), which ensures abundant production throughout the year (Day & Yañez-Arancibia, 1982; Whitfield, 1980b). Phytoplankton is not the sole primary producer, nor is it necessarily the most important. Mangrove leaves, marsh vegetation, grasses, and the phytobenthos also play an important role.

    61In tropical LEEs, these resources display significant variability in time and space (Barett et al., 1978; Deegan & Thompson, 1985). Adaptations developed by species living in these ecosystems are mainly intended to adjust to the unpredictable nature of food availability. Many migratory species, whether marine or continental in origin, optimize the use of lagoonal and estuarine trophic richness by synchronizing their reproduction and migration with periods of nutrient abundance (Yañez-Arancibia et al., 1980; Day & Yañez-Arancibia, 1982).

    62There is little available information on food competition in LEEs. In general, it is believed that the absence of trophic specialization is an efficient way of preventing competition for resources that are abundant but of variable nature and quantity. However, in Southern Africa where detritivores are numerous, it has been shown that competition can be reduced by different mechanisms leading to a certain dietary segregation (Whitfield & Blaber, 1978b). This nonetheless seems to be very rare in many Mugilidae species (Blaber, 1977), which Whitfield (1996) interprets as an indicator that detritivores have access to extremely abundant nutrient resources in estuaries.

    63The most successful trophic strategy in these systems is based on low specialization and strong dietary opportunism. Omnivores (Deegan & Thompson, 1985) and unspecialized predators that can include fishes, crustaceans, molluscs, and insects in their diet are consequently the most numerous species (up to 80%, Albaret, 1994). Crustaceans, particularly Peneidae and Mysidacae shrimp, represent a crucial dietary source for estuarine fishes, particularly young predators (Albaret, 1994).

    64Although they are represented by a relatively low number of species (15 to 30% depending on the case), opportunistic filter-feeders such as Ethmalosa fimbriata that can feed on phytoplankton, zooplankton, and even microbenthos depending on the supply in the ecosystem, often make up the bulk of fish biomass in the LEEs of West Africa (Albaret, 1994).

    65Whitfield (1996) points out the absence of phytoplanktivorous fishes in the estuaries of South Africa where he notes, along with authors such as Blaber (1977), the importance of the detrital path also highlighted by Albaret & Diouf (1994) for the LEEs of West Africa which, according to Diouf (1996), is an addition to primary production-based trophic chains that are usually seen in aquatic systems.

    66We should still note the rarity of linear food chains and of exclusive or highly-determined predator/prey relationships in these ecosystems where, on the contrary, species show low dietary specialization, high adaptability, and even great trophic opportunism.

    Functional role of lagoon and estuarine ecosystems

    Exportation of organic matter and energy

    67The influence of LEEs on the halieutic production of coastal marine areas is well established (Moore et al., 1970; Sutcliffe, 1973; Yañez-Arancibia et al., 1980; Baran, 1995). One of the main paradigms of the ecology of coastal lagoons and estuaries these past three or four decades is that these systems produce an excess of organic matter and nutrients, a good part of which is exported out to sea. This is the so-called “outwelling hypothesis” (Darnell, 1967; Day et al., 1973; Odum et al., 1974).

    68Some studies call this hypothesis into question and suggest on the contrary that organic carbon may be imported into certain LEEs (Hains & Dunstran, 1976; Woodwell et al., 1977). In fact, the direction of the net flow of exchanges between an estuarine environment and a marine coastal zone may depend on local conditions (Day & Yañez-Arancibia, 1982; Day & Yañez-Arancibia, 1985).

    69Fishes in the LEEs play an important role in the flow of energy and nutrients. As secondary consumers, they can constitute energy and nutrient reserves, thus limiting the exportation of nutritional elements. Moreover, through their migrations, they can allow nutrient and energy exchanges with coastal marine systems (Yañez-Arancibia & Nugent, 1977; Kitchell et al., 1979). The exportation of juveniles after growing in lagoon and estuarine environments corresponds to a loss of energy for these systems and an energy gain for the ecosystems receiving them (Thollot, 1992). This energy loss can be considerable. In effect, juveniles migrating out to sea may have multiplied their individual weight by 2,000 during their stay in estuaries and lagoons (Wagner, 1973; Hinchee, 1977). This good growth is also accompanied by high survival rates for these juveniles in lagoon and estuarine ecosystems owing to food abundance and relatively low predation (Day et al., 1989).

    Nursery

    70The importance of estuaries as nurseries for the juvenile forms of many fish species has been seen in diverse latitudes (Johannes, 1978; Beckley, 1984; Ross & Epperly, 1985; Day et al., 1989). In many cases, breeders spawn in coastal areas (Yañez-Arancibia, 1985; Whitfield 1990), then larvae arrive in the estuary through passive transport among plankton or through active swimming. The interpretations put forth to explain the adaptive advantage of this migration generally refer to water turbidity (suspension of mineral and organic particulate matter of a planktonic or detrital nature) and high trophic availability observed in LEEs. Post-larval and juvenile fishes in the estuary thus find favourable trophic conditions that are diverse and suited to their filtration and capture capacities (May, 1974; Miller & Dunn, 1980; Mann, 198; Legett, 1986). The aggregating behaviour of fry with limited swimming and escape capacities, combined with the frequent turbidity of LEE waters that reduce a predator’s distance of perception, increases the probability of escape for prey and thus the overall survival rate for the population. Such a mechanism has been verified experimentally by several authors (Viniyard & O’Brien, 1976; Cyrus & Blaber, 1987a, b, c; Gregory, 1993). Moreover, the abundance of shoals in LEEs restricts the access of large ichthyophagic predators, thereby contributing to less predation pressure on juveniles and small species (Blaber, 1980; Blaber & Blaber, 1980; Kneib 1987). Finally, compared with the marine system, the diversity and structural complexity of habitats in LEEs encourages the concentration and retention of juvenile forms (Fraser & Cerri, 1982; Cowder & Cooper, 1982; Nelson & Bonsdorff, 1990).

    71Such favourable conditions for the nutrition and survival of juvenile forms, combined with the relatively high temperatures often found in LEEs, encourage rapid growth (Potter et al., 1990; Lenanton & Hodgkin, 1985).

    72However, to benefit from these advantages, these forms must have acquired certain ecophysiological adaptations that allow them to thrive in a variable and often unpredictable environment. Thus, the metabolic effort associated with temperature and ion regulation in these tolerant, opportunistic species in the juvenile stage may remain two to three times less than the energy they would consume by actively swimming out of estuaries (Miller et al., 1985).

    73There is broad scientific consensus about the role of LEEs as systems favourable to hosting, survival, and growth of juveniles of many coastal fish species, but many questions remain as to the qualitative aspect of the relationships connecting these species to LEEs and the existence of alternative habitats and nurseries in the coastal environment (Baran et al., 1999).

    74Many species of continental origin also spend part or all of their lives in an estuarine ecosystem. Some, such as Schilbe mandibularis (with a large lagoonal distribution), Parailia pellucida and Schilbe mystus (limited to low salinity and freshwater sectors) can be observed as being notably abundant there (Albaret, 1994). The relationships between fluvial and lagoonal “populations” remain poorly understood, and the nursery role of the estuarine zone is still hypothetical as far as they are concerned.

    Conclusions

    75Contrary to many preconceptions, the composition and general structure of estuarine fish communities display a high degree of stability and robustness at certain time scales (periods of 10 years or more). A connection must be made between this stability and the adaptability of species comprising these communities, the diversity and efficiency of the vital strategies deployed, as well as the existence within these communities of compensatory mechanisms for species (replacement over time of certain species by others that occupy a similar place and role without altering the general structure of the community). The constant availability of varied trophic resources and the sturdiness of food webs (Moyle & Cech, 1982) also constitute a factor contributing to the stability of fish communities. Such long-term stability and robustness nonetheless has limits, which were seen after monitoring the changes that occurred in certain brackish water systems: hypersalinization of the Casamance (Albaret, 1987), isolation of Togolese lagoons from the sea (Laë, 1992a) or, conversely, an increase in exchanges following the opening of a new channel in the Ébrié lagoon (Albaret & Écoutin, 1989), construction of a dam in the upstream section of the Senegal River estuary, etc. These studies show that communities then undergo major changes in species richness, structure, and the mechanisms that govern their functioning. Baran (1995) stresses that tolerance and adaptability are written in the genes of estuarine species (Schachter, 1969; Battaglia & Fava, 1982), but only within the limits of the “natural” environmental variations encountered during the evolution of the species. The example shows that such tolerance and adaptability do not allow species to adapt to disturbances of an anthropogenic nature when these are of unusual scope, duration, or nature (Livingston, 1984).

    76Found at the interface of the continent and the ocean, LEEs are among the environmental systems most affected by anthropogenic activity and global changes. They have been sites of human occupation since prehistoric times. Today, with population growth, these systems are under heavy demand from all directions, often in contradictory ways: fishing, aquaculture, transport, protection of fauna and flora, urbanisation, tourism, various exploitations (e. g. mangrove wood, sand), hydro-agricultural works, etc.

    77While human activities are most often behind the deterioration of LEEs, natural events or changes may also lead to a more or less significant erosion of biological diversity. In fact, quite often the most delicate situations are the result of the combination of the two types of factors and their mutual aggravation. In West Africa, the chronic rainfall deficit in the Sahelian area enters in combination with multiple human interventions, profoundly modifying the biocenoses of estuarine systems without making it possible, at least in the current state of knowledge, to determine the exact degree of “responsibility” of the offending factors.

    Auteur

    • Jean-Jacques Albaret
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    Albaret, J.-J. (2017). Fish communities in estuaries and lagoons. In D. Paugy, C. Levêque, & O. Otero (éds.), The inland water fishes of Africa. Marseille: IRD Éditions. https://doi.org/10.4000/books.irdeditions.25256
    Albaret, Jean-Jacques. « Fish Communities in Estuaries and Lagoons ». In The Inland Water Fishes of Africa, édité par Didier Paugy, Christian Levêque, et Olga Otero. Marseille: IRD Éditions, 2017. doi:10.4000/books.irdeditions.25256.
    Albaret, Jean-Jacques. « Fish Communities in Estuaries and Lagoons ». The Inland Water Fishes of Africa, édité par Didier Paugy et al., IRD Éditions, 2017, https://doi.org/10.4000/books.irdeditions.25256.

    Référence numérique du livre

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    Paugy, D., Levêque, C., & Otero, O. (éds.). (2017). The inland water fishes of Africa. Marseille: IRD Éditions, Éditions Mrac. https://doi.org/10.4000/books.irdeditions.25130
    Paugy, Didier, Christian Levêque, et Olga Otero, éd. The Inland Water Fishes of Africa. Marseille: IRD Éditions, Éditions Mrac, 2017. doi:10.4000/books.irdeditions.25130.
    Paugy, Didier, et al., éditeurs. The Inland Water Fishes of Africa. IRD Éditions, Éditions Mrac, 2017, https://doi.org/10.4000/books.irdeditions.25130.
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